Microwave device



Jan. 2, 1962 M. H. slRvETz 3,015,788

MICROWAVE DEVICE Filed April 15, 1956 2 Sheets-Sheet 1 32'00 H(05RSTEDS) /Nl/ENTOR MARSHALL H. S/RVETZ BY MM TT RNEV Jan. 2, 1962 M.H. slRvx-:Tz 3,015,788

MICROWAVE DEVICE Filed April 15, 1956 2 Sheets-Sheet 2 /M/E/vroAJMARSHALL S/RVETZ BY WM@ United States Patent O 3,015,788 MICROWAVEDEVICE Marshall H. Sirvetz, Cambridge, Mass., assignor to RaytheonCompany, a corporation of Delaware Filed Apr. 13, 1955, Ser. No. 578,0437 Claims. (Cl. S33- 24) aXeS.

same axis independently ofthe direction of propagation as gyrators,isolators, or O neway transmission" lines. When yferrites are used forSuch purposes, it is important that the ferritedevice introduce as4littleloss' as lpossible. This loss is from two or more sources., lThemajor 'portion of lthis loss varies with the applied' static magneticfield, and reaches a relatively high maximum ata'certain applied staticmagnetic field strength." Thisis due to the so-calledvf ferromagneticresonance` effect. ,The applied static magnetic ield strength at whichthis maximum loss occurs varies with the frequencies of the appliedelectromagnetic energy.` Ingeneralgit occurs at lower magnetic forces as4the frequency of the applied'energy is reduced. A second source of lossdecreases lfrom a maximum value at zero magnetic eld `to a very smallvof the energyhas been used in such microwave' devices AH of magneticeld intensities is dependent on the magnetocrystalline anisotropy.

Experimentally, this anisotropy is best observed in single crystals offerrites where the value of the field required for resonance is found todepend on the orientation of the crystallographic axes with respect tothe apr plied static eld. .In themathematical theory'of this effect,anisotropy is expressed'by including in the energy of magnetization aseries of terms depending on the orientation of the magnetization,vrelative to the crystal The first order. term is proportional toananisof tropy coelicient K1; the second, toanother such co-n- `stant,K2, and so forth'.- Frequently, but. no-t always,`

the K1 term is very large comparedto the `higher order terms, and theymay be neglected.,Y 'In the single crystals "it is observed that theresonance region is very narrow compared to those found forpolycrystalline materials.

It is moreover observed, and is explainable on the basis ofthe abovetheory, that the static eld required for resonance varies over a rangeof the order of K/ Ms. when resonance is observedl for A,all possibleorientations of the crystal. Here K is the anisotropy coefficient inergs/cm.3, y

M, is the rsaturation magnetization in c.g.s. units, so that ir/M, isexpressed in oersteds. In polycrystalline ferrites, which may 'beconsidered as collections of crystallites ori.- ented in all directions,one thus observes abroadening of the resonance region, relative to thatfor a single value at the sat-urating magnetic field. The magnetic fieldat which this loss disappears is independent of the frequency of theapplied electromagnetic field. The combined effect of these two .lossesis to give la region of minimum -loss at static magnetic field valuesbelow that for a field giving high losses. Asthe high losses occur withlower static magnetic fields Withvdecreasing frequency, the region oflowloss becomes smallerfwith reduced applied frequency. This effect can beminimized by using materials that have alow saturation magnetizaenergy,the effect produced by the ferrite` is less and becomes increasinglytemperature-sensitive, which is undesirable. From the consideration ofthe variation of the llosses with the applied static magnetic lield, itcan readily be seen that, for devices designed to operate below thefield required for ferromagnetic resonance, a wider useful range ofoperating frequencies and applied static magnetic fields can be obtainedif the range AH of magnetic lield intensity over which the highestlosses takeplace were made narrower.

More generally it can `be said of practically every type of ferritedevice considered in the microwave'engineer-Y ing literature that thequality of the device at low frequencies varies inversely with AH. Thisapplies as well to devices operating at and above the-field required forferromagnetic resonance as to those operating below resonance. Thus atlow frequencies the performance of microwave devices has hitherto beenlimited by the value of AH obtained in available ferrite materials. Thisrange crystal, of the order of K/ MS.

Single crystal ferrites'could obviously be used to obtain theadvantageous narrow resonance region. How# ever, single crystal ferritesare not readily Vavailable commercially. Thus, -it is necessary toobtain the same effect with sinteredA ferrites.

In general this can be accomplishedby working with ferrites of low Curietemperature, since K vanishes near the Curie temperature more rapidlythan doesMsi How-A ever, in this case one incurs the disadvantagesmentioned above, such as low saturation magnetization, poor high powerperformance, and temperature sensitivity. In the compositions which arethe subject of this invention sub- Istantially no decrease in Curietemperaturejis obtained.

It has been shown by such articles as that v.by E.' W. Gorter entitledSome Properties of Ferrit'es in'Connection with Their Chemistry in theProceedings of vthe LRE. for December .1955, vol. 4, page 1945, et.Seq., that ferromagnetic resonance may be pictured as a procf ess inwhich a coherent motion of the elementary magnetic moments on the ionson identical lattice sites takes place. From this pictureV it maybededuced that, if these lattice sites could be occupied by two'different tvpes of ions such that a change in the direction ofmagnetization produced an increase in vthe energy of the magneticmoments associated with one set of ions, which is exactly compensatedfor by a decrease in the energy of the a body of zero anisotropy.

For this, two ferrites having anisotropy energy of the same kind, i.e.,havingV the same form of dependence on orientation, but opposite signare needed. In most cases, K1 is much greater than K2, andonly the K1sneed be 'i l' of opposite sign. The positive` ions, divalent andtrivalent, should occupy. Vthe same. lattice sites in the two ferrites,that is, both should be normal spinels, or both lshould be inversespinels. Fei-rites may be Irepresented by the formula MOFe2O3, with M++representing any one of a number of divalent ions, as M=VNi, Co, Zn, Mn,and so forth.- Still more general formulae may be considered, but theabove suffices for this'discussion.l The spinel ferrite crystal latticecan be represented as made up of a series of cubic units with thepositive metal and negative oxygen ions@ arranged in patterns such thattheoxygen (negative) ions form either tetrahedral or octahedral patternsabout the metal (positive) ions. In the normal spinel-type ferrite, theiron ions are all enclosed in octahedral patterns of oxygen ions, andthe second metal M ions are all associated with tetrahedral patterns ofoxygen ions. In the inverse spinel type, half of the iron ions are intetredral positions, and the rest of the iron ions and all of the secondmetal ions are in oc'tahedral positions. The nature of the M material inpart determines the anisotropy coefficient K1. It has been found thatwhile most of the metal ions tested give a negative K1, cobalt gives apositive, and in magnitude very large, K1. The negative K1 of a metal Mcan be counteracted by the positive K1 of a metal M when ferrites madeof oxides of these metals 'are combined in such proportions as to give aferrite with the formula f (MohqtM'ogrezo,

wherera and l-a are proportionality factors representing the proportionsof the two kinds of metal oxides, which Should be approximately in theinverse ratio of the magnitudes of the coeflicients of the anisotropyfor the two metals. Both ferrites of the mixture should be of the normalspinel lattice pattern, or both should be of the inverse spinel latticepattern to give the best result. The resulting composite ferrite has :analmost zero coefficient of anisotropy and the polycrystalline materialhas a very narrow resonance region, permitting operation in a relativelyrnuch wider region of low loss at lower frequencies than the higheranisotropy ferrites.

Other and further advantages of this invention will become apparent asthe description thereof progresses, ref erence being had to theaccompanying drawings wherein:

FIG. l is an isometric View, partly broken away, of a microwave deviceutilizing the ferrite composition of the invention;

FIG. 2 is a graph of the variation ofloss of a representative ferritewith varying applied magnetic fields as measured in a ferromagneticresonance apparatus;

FIG. 3 is a graph on the loss of a ferrite made according to theinvention with varying applied magnetic fields as measured in aferromagnetic resonance apparatus;

, FIG. 4 is a schematic diagram of a lattice of one type of octantmaking up they unit cells of a normal spineltype ferrite crystal; and

FIG. 5 is a schematic diagram of the lattice of a unit cell Aof a normalspinel-type ferrite with one octant of each type shown with an ionpattern of each type.

In FIG. l there is shown schematically, a typical microwave deviceutilizing ferrites; in this case, a rotational isolator. The microwaveenergy is applied to the rectangular waveguide section in the TEM modein the direction represented by the yarrow 11 with its electrical vectorin the direction indicated by the yarrow 12. The energy is propagatedinto a section of waveguide 13 with circular cross section in the TEMmode through a transition section 14 with no change `in the direction ofpolarization. A thin circular rod 15 of ferrite is supported in adielectric holder 16 formed of a material that presents the minimumdiscontinuity to the microwave energy. An appropriate axial magneticfield is applied to the ferrite 15 by a coil 17. This magnetic field mayalso be applied by a permanent magnet or in any other conarrow 21 inwhich polarization it is propagated down the circular sections 13through a second transition section 22 to a section of rectangularwaveguide 23. The

sheet of glossy material has no effect on this portion input of thedevice and, hence, the device operates as an isolator. It has theadvantage of being a relatively low loss device in the forwarddirection. Theoretical analysis and experiment'show that a device ofthis type has optirnurn performance when operated with applied staticvenient manner. There is mounted in the circular waveguide section 13before the piece of ferrite 15, a sheet 20 of glossy material, with itslongitudinal axis along the -axis ofthe waveguide, and its maintransverse axis in such a direction that energy polarized at rightangles to the incoming energy will be absorbed, while the inmagneticVfield smaller than that required for ferromagnetic resonance.

It isY important that this loss lin such devices be re-k duced as muchas possible. The solid-line graph 30 in FIG. 2 shows how the lossmeasured at 10,000 rnc/sec., plotted vertically along the line 31,varies with the applied magnetic field strength H, plottedhorizontallyvalong the line 32. It will be seen that the loss at nomagnetic eld, represented by point 33, has -a small initial value anddecreases with. increasing field strength to a minimum at point 34 andthen rises with field strength to a maximum at point 35, which in thecase of a representative nickel ferrite was found to occur at 3200oersteds, with a separation AH -at points at Which the loss is half themaximum value of 430 oersteds. The minimum loss 34 occurs at a magneticeld `approximately equal to NMS, where Ms is the saturationmagnetization and N, the demagnetizing factor, depends upon the shape ofthe ferrite and is, in this case,

The region between point 34 and the beginning of the region of maximumloss is the useful region in which the insertion losses are least. Thevalue NMs at the point 34 is independent of frequency. However, themaximum at point 35 varies with the frequency of thev applied microwaveenergy, as shown by the dotted curve 37, representing the same ferriteexposed to the same magnetic eld strengths but in an electromagneticfield of lower frequency. lIt is the same as curve 30, except that theresonance region is shifted to the left while the region between 0 fieldand the beginning of the resonance region tends to remain unchanged, butthe low loss region is considerably narrowed and eventually disappearsas the curve shifts to the left with lower applied frequency.

The effect of narrowing the resonance region by using single Vcrystalsof ferrite, or the sintered-mixed ferrite of the invention, otherparameters remaining the same, is shown in FIG. 3, in which solid-linegraph 40 again shows the variation of loss plotted vertically along theline 41 with varying applied magnetic field H plotted horizontally alongthe line 42. The maximum 43 occurs at the same field strength 3200oersteds, but the Width of the region at the half-loss points isnarrower, 200 oersteds. The dotted curve 44 shows the effect of lowering the frequency by the same amount, as in FIG. 2. It will be notedthat the maximum 45 has shifted to the left as before, but does notencroach as much on the region of low insertion loss between 0 andpoints 45 and 47 as with the ferrite of FIG. l.

The lattice of ay ferrite is of interest here. Such a lattice is of thespinel type and is shown in FIGS. 4 and 5. This lattice may be describedby dividing a unit cell, as shown in FIG. 5 and designated by thereference numeral 70, into eight equal cubes, or octants 71. One ofthese octants is shown in FIG. 4. These octants are This metal ion canbe said to occupy a tetrahedral posi-v tion. The other type of octant isshown at the front upper right of the unit cell 70 in FIG. 5. It alsohas four positive ions 72cm the alternate corners and four negative ionshalfway out on the opposite halves of the body diagonals. In addition,there are four positive ions designated by small shaded circles 76. Itwill be noted that each positive ion4 76 of the second type issurrounded by six negative ions 73, forming an octahedron, such as 6 ofanisotropy of Vopposing sign each present in inverse proportion to itscoeiicient of anisotropy.

3. A microwave device including a ferritev having substantially a zerocoefiicient of anisotropycomposed of two ferrites, one of said ferritescomposed of a mixture of Fe2O3 and CoO, and the other composed of Fe203and an Yoxide selected from the group consisting of nickel, zinc, andmanganese, said ferrites having the same type of spinel crystalstructures and coefficients of anisotropy of opposing sign each presentin'inverse proportion to its coeicient of anisotropy.

4. A microwave device including a ferrite having substantially a zerocoeficient of anisotropy having the composition (MO) (MO)1 FezOg where Mis cobalt and MY is nickel the oxide of which forms a yferrite with ironoxide having a negative coetiicient of anisotropy and a l isapproximately inversely proportional to the coefficient the six ions73a, b, c, d, e and f, joined by the light lines 77. This is referredtohereinafter as an octahedral posi'- tion. In a unit cell, such as thatshown in FIG. 5, ,those A octants sharing an edge only are of the sametype and those sharing a face are of different types. In the unit cellthere are enough ions of these types to make up eight molecules of thespinel compound.

In a normal spinel-type of ferrite of the formula MFe2O4, where M standsfor a positive divalent ion, all the tetrahedral positions are occupiedby the divalent ions and all the octahedral positions by the iron ions.That is, ions 72 of FIGS. 4 and 5 would be M'ions, and the ions .76would be iron ions. In the inverse spinel lattice, all the tetrahedralpositions are occupied by iron ions, and the octahedral positions areshared equally between the two types of positive ions.

.Nickel and cobalt oxides are particularly adapted to making thecomposite ferrites of the invention as they both have the same type ofspinel lattice and have Curie vpoints in the same order of relativelyhigh magnitude. The coefhcient of anisotropy K1 of nickel ferrite,

(NiO)Fe2O3 is approximately -5 l04, While that of cobalt ferrite(CoO)Fe2O3 is approximately +2 106. Optimum results Wereobtainedvwhenthe proportionality factor a Was taken as 0.025, giving the formula(COO)0,025(NiO) 975Fe2O5. This is in agreement with predictions based onthe single crystal data.l Theferrite body of the invention may beprepared by any of the known methods of preparing single ferrite bodies.It

will be apparent that this technique is not limited to nickel and cobaltferrites but can be applied to obtain narrow loss regions in ferriteswhenever two single ferrites possessing `anisotropy coeiiicients ofopposite sign are mixed in appropriate proportion as described above.

This invention is not limited to the particular details of construction,materials and processesl described, as many equivalents will suggestthemselves to those skilled in the art. It is accordingly desired thatthe appended claims be given a broad interpretation commensurate withthe scope of the invention Within the art.

What is .claimed is:

l., A microwave device including a ferrite having substantially a zerocoefficient of anisotropy composed of two ferrites having the same typeof spinel crystal structures and coeiiicients of anisotropy of opposingsign each present in inverse proportion to its coefficient of anisotropysaid ferrites consisting of a mixture of CoO and an oxide of one of theelements selected from the group consisting of nickel, zinc, andmanganese, which forms ferrites with iron oxide.

2. A microwave device including a ferrite having substantially a zerocoefficient of anisotropy composed of two ferrites having thecomposition of isio--coO--Fem3 having the same type of spinel crystalstructuresin which the positive ions occupy the same sites, andcoetiicients of anisotropy of a ferrite formed with cobalt oxide, 1-a

is'inversely proportional to the Icoefficient of anisotropy l 'of alferrite formed with M'O which coefficient is opposite in sign to that ofthe ferrite formed with MO.

A microwave device including a ferrite having substantially a zerocoetlicient of anisotropy having the composition (MO), (MO)1 Fe2O3 whereM is nickel and M vis cobalt the oxideA of which forms a ferrite withiron oxide having a positive coeflicient of anisotropy and a isinversely proportional to the coeicient of anisotropy of a ferriteformed with nickel oxide, l-a is'inversely proportional to thecoefficient of anisotropy of a ferrite formed with MO which coefficientis lopposite .in sign to that of the ferrite formed with MO, and Fe2O3is an iron oxide. 1

l6. A microwave device including a ferrite having substantially a zerocoetcient of anisotropy having the com- POSition ((300)0.025 (Nio)n.9'15F6203 7. An isolator device for electromagnetic energy, comprisingwaveguide means for guiding such energy, magnetic means for establishinga magnetic ield within said vwaveguide means, and an insert memberWithin such References Cited in the tile of this .patent UNITED STATESPATENTS 1,976,230 Kato et al Oct. 9, 1934 2,640,813 Berge June 2, 19532,656,319 Berge Oct. 20, 1953 2,723,239 Harvey Nov. 8, 1955 2,736,708Crowley Feb. 28, 1956 FOREIGN PATENTS 1,071,068 France Mar. 3, 19541,086,346 France Aug. 11, 1954 1,091,735 France Nov. 3, 1954 697,059Great Britain Sept. 16, 1953 f 717,269 Great Britain Oct. 27, 1954521,341 Belgium Jan. 9, 1954 524,097 Belgium Nov. 30, 1953 OTHERREFERENCES Economos: I. Amer. Ceramics, July 1955, pp.241244.

Kawai: J. Soc. Chem. Ind., Japan. vol. 37, Nov. 4, p.

J. Institute of Electrical Engineers, Japan, November 1937, pp. 4, 5;Iune 1939, pp. 277, 279, 281.

(Other references on following page) 7 8 OTHER REFERENCES Neel: Z.Anorg. Chem., vo1`. 262, p. 178 (1950). Weil: compres Rendus, vol. 234,pp. 1351, 1352 PhYSlcal Revlew "O1- 99 NO- 6 Sept, 15 1955 v pagesSpecification CatalogV Na 54, pp. 13 M136, pub. by Journal ofElectronlcs, vol. 1, No. 1, July 1955,'

LT.Bakerchem.C0.,Phi11ipsburg,N.J.,Apri11954. 5 Pages 64-77-

7. AN ISOLATOR DEVICE FOR ELECTROMAGNETIC ENERGY, COMPRISING WAVEGUIDEMEANS FOR GUIDING SUCH ENERGY, MAGNETIC MEANS FOR ESTABLISHING AMAGNETIC FIELD WITHIN SAID WAVEGUIDE MEANS, AND AN INSERT MEMBER WITHINSUCH MAGNETIC FIELD COMPRISING A BODY OF THE COMPOSITION NI1-XCOXFE204,IN WHICH "X" IS APPROXIMATELY INVERSELY PROPORTIONAL TO THE COEFFICIENTOF ANISOTROPY FORMED WITH COBALT OXIDE AND 1-X IS INVERSELY PROPORTIONALTO THE COEFFICIENT OF ANISOTROPY OF A FERRITE FORMED WITH NICKEL, WHICHCOEFFICIENT IS OPPOSITE IN SIGN SO THAT OF THE FERRITE FORMED WITHCOBALT, WHEREBY THE LOSSES IN SUCH ISOLATOR DEVICE ARE SIGNIFICANTLYREDUCED AS A RESULT OF THE SUBSTANTIALLY ZERO MAGNETOCRYSTALLINEANISOTROPY OF SAID INSERT MEMBER.